This invention relates to a gauge for measuring and/or exhibiting the liquid level in a container of alkaline liquid. It is more particularly concerned with a glass component for such gauge which has a strong resistance to alkali attack.
Traditionally, gauges of this sort have been used to observe the water level in boilers for heating and power generating plants. The observation window in such boiler may take the form of a flat, glass block or bar, and is frequently called a sight glass. The inside surface of such sight glass is necessarily exposed to a corrosive environment provided by a combination of high pressure steam and hot water which tends to be of an alkaline nature.
Recently, a liquid level monitoring system, based on fiber optic principles, has been proposed for use in truck radiators and the like. In this system, light is transmitted to and from a glass sensor via flexible fiber optics. The sensor may be a glass prism which, when covered with liquid, refracts into the liquid and thus dissipates any light received within the prism. Conversely, when the prism is uncovered, as by a drop in liquid level, input light is reflected back to a return or pickup fiber optic whereby a warning signal is activated. The glass prism must have a high resistance to corrosive alkaline attack so that a relatively clear surface, needed for effective operation of the system, is maintained over an extended period of time.
Conventional glasses deteriorate rapidly in an alkaline environment. Consequently, mica has been substituted as a visual medium because it is one of the few transparent materials other than glass that will withstand such alkaline attack to any degree. However, mica is a relatively expensive material for this use. Also, the resistance to alkaline attack is erratic, that is, it may vary greatly from one type of mica to another, or even between lots of the same type. Accordingly, extensive studies have been made in a search for glasses having a greater resistance to alkaline attack.
It has been proposed to incorporate substantial amounts of zirconia (ZrO.sub.2) in a silicate glass to enhance the resistance of such glass to alkaline attack. This is disclosed for example in "The Properties of Glass" by G. W. Morey published by Reinhold Publishing (1938). One such commercial glass is composed essentially of 12.5% R.sub.2 O, 16.5% ZrO.sub.2 and 71% SiO.sub.2. Such glasses are difficult to melt and work, and still fail to provide the degree of resistance to alkaline attack required in many applications. A more easily melted Na.sub.2 O-B.sub.2 O.sub.3 -Al.sub.2 O.sub.3 -SiO.sub.2 glass has been developed for production of commercial gauge glasses, but the alkali resistance thereof is not as great as would be desired.
The mechanism of alkali attack on a silicate glass involves the reaction of the hydroxyl ion with the glass network: ##STR1## The attack is generally manifested by a weight loss and a deterioration of surface appearance. The action of steam and hot water on glass involves a similar mechanism. However, the initial step is a dissolution of alkali cations from the glass. This creates an alkaline solution at the glass surface, which then attacks the glass by the above reaction. The reaction between a soda-containing, silicate glass and water would be: ##STR2## Therefore, a glass with good alkali durability should also be resistant to attack by hot water and steam.
One method used to determine resistance of a material to alkaline attack is to treat a polished plate in 5% sodium hydroxide solution at 95.degree. C. for 6 hours. The loss of weight is determined and reported as milligrams per square centimeter of surface area. Appearance change is also noted. Materials that show promise in this test may then be evaluated in hot water and steam at 215.degree. to 365.degree. C. for 24 to 28 hours. Alternatively, they may be subjected to steam and hot water containing 0.5% sodium sulfate and 0.11% sodium carbonate at 215.degree. C. for 3 hours in a test known as the Pennsylvania Railroad Test.
Copending application Ser. No. 273,435, filed July 20, 1972 by one of us, now U.S. Pat. No. 3,804,646, discloses silicate glasses characterized by very high elastic moduli, a property which makes such glasses particularly useful for producing fibers, ribbons and other glass forms that impart strength to plastic composites. These glasses are composed essentially of, in mole percent, 25-60% SiO.sub.2, 5-30% Al.sub.2 O.sub.3, 5-40% MgO, and 8-40% of a high field strength modifier of the group TiO.sub.2, Ta.sub.2 O.sub.5, La.sub.2 O.sub.3, Y.sub.2 O.sub.3, CaO and ZrO.sub.2.